Chronic Erosive Synovitis can be induced experimentally in rats by systemic administration of aqueous suspensions of cell walls from certain bacteria, such as Group A streptococci and lactobacilli. Host genetic and sex-linked hormonal factors, as well as cell wall factors, influence incidence and severity of acute and chronic disease. Arthritogenic cell walls are lysozyme resistant and are rich in rhamnose. The development of chronic arthritis in susceptible rat strains is dependent, in variable degrees, upon 1) deposition and persistence of the cell walls in synovial tissues, and 2) a relative inability to neutralize the proinflammatory properties of the cell walls. Studies of the detailed mechanisms underlying the induction of chronic arthritis are continuing.